This application is based on Korea Patent Application Nos. 2000-56246 filed on Sep. 25, 2000, and 2001-36767 filed on Jun. 26, 2001 in the Korean Industrial Property Office, the contents of which are incorporated herein by reference.
The present invention relates to a positive active material for a rechargeable lithium battery and a method for preparing the same, and more particularly, to a positive active material for a rechargeable lithium battery having improved thermal safety, and a method for preparing the same.
Rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated for positive and negative active materials. For an electrolyte, an organic solvent or polymer is used. A rechargeable lithium battery produces electric energy as a result of changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of lithium ions.
For the negative active material in a rechargeable lithium battery, metallic lithium was used in the early days of development. Recently, however, carbon materials, which intercalate lithium ions reversibly, are extensively used instead of metallic lithium due to problems of high reactivity toward the electrolyte and dendrite formation of the metallic lithium. With the use of carbon-based active materials, the potential safety problems which are associated with the metallic lithium can be prevented while achieving relatively high energy density, as well as a much improved cycle life. In particular, boron may be added to carbonaceous materials to produce boron-coated graphite (BOC) in order to increase the capacity of the carbonaceous materials.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples include LiCoO2, LiNiO2, LiNi1-xCoxO2 (0 less than x less than 1), and LiMnO2. Manganese-based materials such as LiMn2O4 and LiMnO2 are the easiest to prepare, are less expensive than the other materials, and are environmentally friendly. However, manganese-based materials have relatively low capacity. LiNiO2 is inexpensive and has a high capacity, but is difficult to prepare in the desired structure and is relatively less stable in the charged state causing a battery safety problem. LiCoO2 is relatively expensive, but widely used as it has good electrical conductivity and high cell voltage. Most commercially available rechargeable lithium batteries (at least about 95%) use LiCoO2 as the positive active material.
Although LiCoO2 exhibits good cycle life characteristics and good flat discharge profiles, there are still demands to improve electrochemical properties such as good cycle life and high power density.
One way to satisfy such a demand is to substitute a part of the Co from LiCoO2 with other metals. Sony studied LixCo1-yAlyO2 by doping about 1 to 5 percent by weight of Al2O3 into LiCoO2. AandTB (Asahi and Toshiba Battery Co.) studied a Sn-doped Co-based active material by substituting a part of Co from LiCoO2 with Sn.
As electronic products become smaller in size and lighter in weight, a lithium rechargeable having improving electrochemical properties such as high capacity and long cycle life are desired.
It is an object of the present invention to provide a positive active material for a rechargeable lithium battery cell exhibiting longer cycle life characteristics and enhanced discharge voltage.
It is another object of the present invention to provide a method for preparing the same.
In order to achieve these objects, the present invention provides a positive active material for a rechargeable lithium battery cell in which the active material includes a core including a lithiated compound and a surface-treatment layer formed on the core. The surface-treatment layer includes a core element-included oxide, or a core-element-included hydroxide, oxyhydroxide, oxycarbonate, hydroxycarbonate or a mixture thereof. The lithiated compound may be made of secondary particles of an average size larger than or equal to 1 xcexcm and smaller than 10 xcexcm in diameter. The secondary particles are made of an agglomeration of small primary particles of an average size of approximately 1 to 3 xcexcm in diameter.
The present invention also provides a method for preparing the same. In the method, the lithiated compound is coated with an organic solution or an aqueous solution including a coating-element source, and the coated lithiated compound is heat-treated.